He said he saw the student on Friday morning wearing a black trench coat, military boots and a t-shirt saying Born To Kill.

Galveston County judge Mark Henry, the county's highest elected official, said that several explosives were found at the high school and in nearby areas.

Fearing that more explosives may be planted around the area, he urged locals to call emergency services if they saw any suspicious items.

The gunman opened fire inside the school at about 7.30am, around the time school was about to start for the day, according to Joe Giusti, a Galveston County commissioner.

A student, Angelica, said she heard a fire alarm go off and, about two minutes after students began evacuating, she heard gunshots ring out. Other students said they heard what sounded like explosions.

"[Some students] were, like, 'run, run, there's gunshots!' Nobody believed them until the gunshots got closer and then we all started running," she told MSNBC. "Everybody ran to the nearest gas station and waited for their parents there."

Her mother, Angie Martinez, told MSNBC that he daughter called her during the fire alarm as the chaos broke out.

"I said, 'calm down', I said, 'find a hiding place'. She told me they were outdoors and a lot of students were running. I said, 'well, run to the bushes'. I said, 'don't get off the phone'."

Giusti said one police officer who worked for the Santa Fe school district as a school resource officer was injured in the shooting. The gunman was uninjured, he said.

Three people were transported to University of Texas Medical Branch in Galveston: one middle-aged male in a critical condition with gunshot injuries to his upper arm, one woman is good condition with a gunshot wound to the leg and one male student in a good condition with a gunshot wound to the leg.

The Harris County sheriff's office said its deputies were assisting with a "multiple-casualty incident" that was "no longer an active situation".

Student Dakota Shrader said she initially thought it was a regular fire drill until she heard three shots and the sound of an explosion.

"All these teachers were telling us to run, run, go, go," she said, through tears. "This is a place where we're supposed to feel safe, nobody should have to go through this, nobody should have to feel that pain."

Aerial video outside the school showed police escorting lines of students out of the building and then searching them for weapons as many police cars and at least two ambulances with lights flashing stood by.

A sophomore, named only as Nikki, told ABC13 that: "Someone had walked in with a shotgun and a girl got shot in her leg".

Speaking at an event at the White House, US President Donald Trump said the country was dealing with "sadness and heartbreak".

"This has been going on too long in our country, too many years, too many decades now," he said.

"We grieve for the terrible loss of life and send our support and love to everyone affected by this absolutely horrific attack. We're with you in this tragic hour and we will be with you forever."

He said his administration "is determined to do everything in our power to protect our students, secure our schools and to keep weapons out of the hands of those who pose a threat to themselves and to others".

The latest such incident at a US school underscored a national debate over gun control and gun rights that has intensified after a student killed 17 students and staff on February 14 at Marjory Stoneman Douglas High School in Parkland, Florida.

Days after the Parkland shooting, Trump said that elected officials should be ready to "fight" the powerful NRA lobby group. Early this month he embraced that group, telling its annual meeting in Dallas "your Second Amendment rights are under siege" and, as long as he was President, their guns would never be taken away.

The Second Amendment of the US Constitution protects the right to bear arms.

No major federal gun controls have been imposed since Parkland, though the administration is pursuing a proposed regulatory ban on "bump stocks", which enable a semi-automatic rifle to fire a steady stream of bullets.

Just last month, students from Santa Fe High School had staged a walkout in solidarity with Marjory Stoneman students who had organised a national day of protest against gun violence.

with Reuters